During converting operations, e.g. fiber web processing machine operations, of paperboard or paper, such as decurling, sheeting, cutting, creasing, pigment coating, polymer coating, printing, decurling, varnishing or embossing, structural changes such as delamination and pigment coating cracks can initiate. Delamination can also occur during the production process of paperboard or paper, for example during wet pressing, drying or calendering. Delamination occurs when the applied load on the paperboard or paper exceeds the strength of the material. The fibers in the paperboard or paper will then be locally separated; this will create a new configuration of the fiber network and new free surfaces inside the material. The end user may then notice a reduced internal strength as the paperboard or paper is exposed to a new set of external loads. If the paperboard consists of several layers, it is also possible that the layers will be separated; this which also results in structural changes such as delamination and decreased internal strength. Internal strength reduction can cause runability or quality problems for the next fiber web processing step, e.g. printing, creasing, folding and lamination. If the paperboard or paper is pigment coated and the applied external load is too large, the coating can crack which will create an uneven surface of the product. Coating cracks strongly influence the printing properties of the product, which are very dependent on a smooth surface.
Both delamination, coating cracks and other structural changes such as light scattering changes are often difficult to detect visually or with standard laboratory test methods. In the diploma work “En kritisk granskning av metoder för att detektera delaminering i papper” by Johan Persson, {dot over (A)}BO AKADEMI, KEMISKA TEKNISKA FAKULTETEN, 2000, the insufficiency of different methods for measuring delamination is discussed. One method used for measuring strength properties in the thickness direction, often referred to as z-direction (ZD), of paperboard or paper products is z-strength testing. Another method is Scott-Bond. However, these methods are destructive and do not indicate where delamination or weakened zones are located because the commonly used test frequency is very low. They only show that the strength of the paperboard or paper has decreased. There are also several other methods that can be used, i.e. peeling, light scattering or permeability tests like Gurley. Common among all those methods is that they are not sensitive enough to measure millimeter size delaminations or small coating cracks. Today, structural changes are commonly detected visually during regular production; this approach is subjective as it depends on the person inspecting the material. The inspected part of the material is also very limited, i.e. damaged material can pass the process.
Traditional treatment and analysis techniques are based on the evaluation of elastic properties by means of in-plane ultrasonic waves applied to paper sheets or webs. One example is described in WO03095744.
WO03095744 describes a method and an apparatus for online monitoring of a paper sheet during production. According to the method, a laser generator generates and directs a laser beam at the paper sheet as it travels through the production process. As the laser beam impinges the paper sheet, in-plane ultrasonic waves are generated. A second laser generator is used to generate and direct a second laser beam at the paper sheet. The second laser beam is reflected from the paper sheet with a modulated characteristic associated with the sonic waves propagated in the plane. An interferometer is used to measure at least a part of the second laser beam as it is reflected from the paper sheet. The interferometer may then send a signal to a distributed control system, which may implement an action such as changing production process parameters.
The method according to WO03095744 thus utilizes in-plane propagation of sound waves in paper sheets. However, the article On the verification of the applicability of the orthtropic plate wave theory to paper, T. Pettersson, J. Anttila, Ultrasonics 39 (2002) 617-622, describes an alternative interpretation of the theory on sound wave propagation through a porous material such as paper. According to the article, when applying ultrasonic waves to a paperboard the so-called X mode wave was found to propagate in the thickness direction (ZD) of the paperboard; this is contrary to previous beliefs. According to the invention, this finding on the direction of propagation of the X mode wave is utilized to achieve a new and improved method for detection of structural changes in a porous material such as paper, paperboard or board.